Initially, the optical disc for the magneto-optical disc will be described below.
In the magneto-optical disc, recording signals are recorded by perpendicularly magnetizing the magnetic film in an upper direction or in a lower direction in accordance with recording signals. Reproduction of the magneto-optical disc is carried out by irradiating linearly polarized laser beams onto the disc. With respect to a reflected light of linearly polarized laser beams irradiated onto the magneto-optical disc, its plane of polarization is rotated in dependency upon the fact that the direction of magnetization is changed upwardly or downwardly by the so-called Kerr effect. In the optical system for the magneto-optical disc, change (fluctuation) of the plane of polarization of reflected light is converted into change of light intensity by light detection element, e.g., polarization beam splitter, etc. Photo-detector detects such change of the light intensity as light current.
For the photo-detector, photo-diode, e.g., PIN photo-diode or avalanche photo-diode, etc. is used. By allowing this light current to be passed through current/voltage converter, reproduction signal is obtained.
Explanation will be given in more practical sense. Since a reflected light from the magneto-optical disc is such that since quantity of rotation of the plane of polarization is extremely small, common mode (in-phase) noise component is canceled so that S/N ratio can be improved. The reflected light from the magneto-optical disc is caused to undergo rotation of the plane of polarization, e.g., by half wave plate thereafter to convert change of plane of polarization into light intensity modulated two light beams which are opposite to each other in phase through a light detection element. Changes of light intensity of these light beams are respectively detected by photo-detectors to obtain a magneto-optical reproduction signal by differential component (difference).
A more practical example of a conventional circuit configuration for such a magneto-optical signal reproduction will be described with reference to FIG. 1.
Two light beams which have been intensity-modulated in the state opposite to each other in phase are incident to photo-diodes 80, 81 shown in FIG. 1. As a result, electron (positive) hole pairs are produced in proportion to incident light quantity so that current is caused to flow. Thus, two current signals modulated in the state opposite to each other in phase are respectively taken out from the anode sides (or the cathode sides). In taking out detection output currents from these photo-diodes 80, 81, in order to improve the response speed or the linearity, inverse biases are respectively applied to the terminals to reduce capacity between electrodes.
Output currents from the photo-diodes 80, 81 are respectively passed through capacitors 82, 83 so that DC component is eliminated, and are then converted into voltage signals by current/voltage converters 84, 85. The current/voltage converters 84, 85 respectively output voltage signals of phases opposite to each other to the inverting terminal (-) and the non-inverting terminal (+) of a differential amplifier 86. The differential amplifier 86 removes common mode (in-phase) noise component of the delivered signal to take out a differential (difference) component to thereby obtain a reproduction signal.
The detection technology for light signal by the differential detection system will now be described as the related technology.
Initially, FIG. 2 shows a conventional single detection system, and this system is a method of taking out electron/(positive) hole pairs produced by incident light to photo-diode 80 from one end of the photo-diode 80, i.e., either one of the anode and the cathode. Accordingly, this method only takes out one of electron hole pairs.
On the contrary, in accordance with the differential detection system, as shown in FIG. 3, for example, output currents of phases opposite to each other corresponding to incident light are taken out from the anode side and the cathode side of the photo-diode 80. The output currents of phases opposite to each other are respectively delivered to current/voltage converters 89, 90 through capacitors 87, 88. Thus, the current/voltage converters 89, 90 provide voltage signals of phases opposite to each other. Respective voltage signals of phases opposite to each other are delivered to a differential amplifier 91. The differential amplifier 91 takes out a differential component to thereby obtain a reproduction signal. Signal quantity at this time becomes equal to a value twice greater than the signal quantity by the conventional single detection system.
In this case, although there are used two current/voltage converters 89, 90 of which number is twice greater than that of the single detection system, since noise produced in the current/voltage converter is random noise, noise generated from the current/voltage converter in a final output signal of the circuit of FIG. 3 becomes equal to a value of .sqroot.2 times. As a result, the noise level of the current/voltage converter with respect to the signal level can be reduced to 1/.sqroot.2.
An example of a magneto-optical signal detection circuit to which such a differential detection system is applied is shown in FIG. 4. The circuit of FIG. 4 is constituted by using two circuits of the differential detection system of FIG. 3, and reference numerals of corresponding portions of FIG. 3 with primes a, b are respectively attached to the portions of these two circuits of the differential detection system, and their explanation will be omitted.
In FIG. 4, output signals from differential amplifiers 91a, 91b are delivered to a differential amplifier 92, thereby making it possible to reduce the noise level of the current/voltage converter with respect to the signal level to 1/.sqroot.2.
Meanwhile, in such conventional magneto-optical signal detection circuit, since reflected light from the magneto-optical disc is very weak, noises that the current/voltage converters 89a, 89b, 90a, 90b generate are applied to a reproduction signal. For example, in such a digital signal reproducing apparatus to obtain a reproduction signal from the magneto-optical disc as stated above, error rate is deteriorated (lowered) by application (superposition) of noise onto the reproduction signal.
In recent years, studies of realization of short wavelength of the light source for irradiation onto the optical disc are energetically carried out with a view to realization of high density of the optical disc. However, the photo-diode for detecting reflected light from the optical disc has a tendency such that the light receiving sensitivity of the photo-diode lowers with shortening of wavelength. As a result, the influence of noise of the current/voltage converter has been a pressing or urgent problem.
Moreover, in accordance with the circuit of FIG. 4, the noise level of the current/voltage converter with respect to the signal level can be reduced to 1/.sqroot.2, but two current/voltage converters and two differential amplifiers are increased as compared to the circuit configuration shown in FIG. 3, resulting in large circuit scale. Enlargement of the circuit scale leads to increase in the power consumption and cost of the circuit.
A circuit for taking out differential component between detection signals from respective light receiving elements 80, 81 as shown in FIG. 3 is adapted to take out a magneto-optical signal from a reflected light from the magneto-optical disc. On the contrary, there is the technology for taking out light signals from the optical disc recorded by phase pits or reflection factor change.
The recording by phase pits mentioned here is the system of forming uneven phase pits on the optical disc in accordance with information to carry out recording. This recording system is employed in the reproduction only type optical disc or the write once type optical disc, etc. Moreover, in recording by reflection factor change, the physical state of the recording layer onto which laser beams are irradiated is caused to be changed, e.g., from the amorphous state to crystalloid, etc. so that recording of information is carried out. The reproducing unit carries out reading of information in dependency upon difference of quantity of reflected light of laser beams which has reflected the state change of the optical disc corresponding to presence or absence of recording of information. This recording system is employed in the write once type or rewritable type optical disc, etc.
In the reproduction of the optical disc of the recording system by phase pits or reflection factor change, as apparent from the feature of the system, light signals which have been caused to undergo light intensity modulation in the in-phase state are incident to two photo-diodes. In view of this, an adder 95 is provided as shown in FIG. 5 to carry out reproduction of in-phase light signal by adding output signals from current/voltage converters 84, 85. Namely, from respective anode sides (or respective cathode sides) of the photo-diodes 80, 81, current outputs which are both in phase are obtained. These current outputs are converted into voltage signals at current/voltage converters 84, 85 to send them to the adder 95. At this time, no signal appears on the output terminal of the differential amplifier 86.
Also in the conventional magneto-optical recording/reproducing apparatus, in order to, e.g., realize reproduction of address information, etc. recorded by phase pits in advance on the magneto-optical disc, or compatible reproduction of the reproduction only disc recorded by phase pits, there are employed many configurations in which not only the differential amplifier 86 but also the adder 95 are provided.
Meanwhile, when attempt is made to realize both the magneto-optical signal detection and detection of signal recorded by phase pits or reflection factor change, differential amplifier 86 and adder 95 are required as shown in FIG. 5, resulting in the problem that the circuit configuration becomes complicated.
In addition, in order to realize high density of the optical disc, also in the case of attaining reduction of noise of the current/voltage converter by the above-described current addition system, it is expected to realize, by simple circuit configuration, not only reproduction of the magneto-optical signal but also reproduction of signal recorded by phase pits or reflection factor change.
This invention has been made in view of actual circumstances as described above, and its object is to provide a light signal detection circuit which can prevent the influence of noise even when the optical disc is caused to have high density while suppressing the circuit scale so that it becomes small, and which can reproduce information without depending upon the kind of the optical disc.